1. Field of the Invention
The present invention is related to an information recording apparatus, an information recording method and an information recording medium, which record and reproduce information by using light.
2. Description of the Related Art
Optical disks may constitute the following major features. That is, semiconductor lasers can be used as light sources, recording media (disks) can be unloaded from recording/reproducing apparatus, and cost per bit of recording media is inexpensive. As a consequence, high density and high-speed recording/reproducing operations are desirably realized without loosing these features in optical disk apparatus.
To increase recording capacities in these days, the optical disks are known in this field, on which information is recordable on transparent organic materials in a three-dimensional direction involving a thickness direction. However, in the optical disks using two photon absorptions, large-scaled lasers on single-crystal recording media are required. In the optical disks using optical polymerization, storage stabilities and recording sensitivities are deteriorated. In hologram recording optical disks, these optical disks can be hardly loaded/unloaded.
In order to realize the recording operations in high speeds, there is limitation in such a case that the rotation speeds of optical disks are increased. Since the optical disks own such feature that they can be loaded/unloaded, and plastic base plates are also employed in recording media to realize low cost, there is no way capable of avoiding upper/lower oscillations of outer circumferential portions of these optical disks and decentering of these optical disks. When the rotation speeds of optical disks are increased, the upper/lower oscillations and decentering of these optical disks become high frequencies, so that auto-focusing operations and tracking operations can be hardly followed. As a consequence, if the rotation speeds are increased at higher levels than a certain level, then parallel recording operations must be taken into account. As to using the parallel recording system, multi-beam recording systems have been conventionally proposed.
On the other hand, the following experimental results have been reported with U.S. Pat. No. 3,801,966 and M. Terao et al., “Highly Sensitive Amorphous Optical Memory” Supplement to the Journal of the Japan Society of Applied Physics Vol. 42, pages 233 to 238 in 1973). That is, both a photo-conductor and a phase-change recording film are sandwiched by transparent electrodes, when a light is irradiated while a voltage is applied by way of these transparent electrodes, photo-currents are multiplied, so that information can be recorded by using a weak laser light by 2 digits, as compared with that obtained when only the laser light is irradiated.
In the above-described parallel recording system using the multi-beam recording operation, after information has been read/written over one circumferential track, a large track jump is required over a long distance corresponding to a total beam number. Also, this parallel recording system owns another problem that formats of an optical disk are different from each other every total beam number. This problem may be solved by such a way that the respective optical spots produced by collecting the multi-beams may own focal points on the different layers of a multilayer recording medium. To this end, an optical axis involving setting angles of lasers is inclined, or a lens is inclined. For example, in such a case of a phase-change write-once type 4-layer recording medium in 4-beam simultaneous recording system, optical absorptions and thermal diffusion are designed in such a manner that recording sensitivities of 4 layers may become substantially identical to each other. However, in this recording system, in order to prevent interlayer crosstalk, since a layer interval is large, i.e., approximately 20 μm, an optical axis must be largely inclined, as illustrated in FIG. 9. Thus, there are problems that laser beams can be hardly collected due to aberration, and a lens impinges on an optical disk. For instance, in the case that a semiconductor array laser whose beam interval is 100 μm is employed, an optical spot interval becomes approximately 10 μm based upon an NA ratio of a collimator lens to a focusing lens. As a result, if a layer interval of an optical disk is equal to 20 μm, then a tangent value of an inclination becomes 2. Therefore, there are problems that the laser beams can be hardly collected due to aberration and the focusing lens impinges on the board. As a consequence, a plurality of individual lasers must be adhered to each other with securing stepped portions, by which precision and manufacturing yield of such a semiconductor array laser can be hardly increased. Also, since the respective layers of the optical disk own optical absorptions, recording sensitivities thereof are lowered, and thus, there are some possibilities that recording speeds are delayed due to a lack of laser power. It should be noted that, in FIG. 9, a reference numeral 91 shows a base plate, 92 shows a recording layer and 93 indicates a spacer layer, and further, 94 denotes a reflection layer.
As described in U.S. Pat. No. 3,801,966 and “Highly Sensitive Amorphous Optical Memory” written by M. Terao et al., Supplement to the Journal of the Japan Society of Applied Physics Vol. 42, pages 233 to 238, in 1973, since the recording sensitivity is extremely high, the high speed recording operation can be carried out by employing the above-described arrangement without a lack of laser power. Also, in such a case that an optical disk is made of multiple layers to realize a large storage capacity, even when laser light is irradiated onto adjoining layers, if a voltage is not applied thereto, then information is not recorded/reproduced. As a result, layer selective characteristics may be obtained. However, since an optical absorption is slightly required, the laser light can be hardly reached to deeper layers due to the optical absorption.